CYCLOHEXANE ADSORPTION ON CLEAN AND O-COVERED NI(111) - IMPLICATIONS FOR C-H BOND ACTIVATION

The adsorption of cyclohexane on clean and oxygen-modified Ni{111} sur faces has been studied by reflection-absorption infrared spectroscopy (RAIRS). Adsorption on the clean surface proceeds via two-dimensional island growth in equilibrium with a random gas phase adsorbed on the b are surface and on top of the islands. The adsorbed cyclohexane exhibi ts a site symmetry of C-3v which persists through to the multilayer re gime. Adsorbed molecules in the first layer exhibit a broadened and do wnshifted nu(CH) stretching vibrational band which has become the sign ature for cyclohexane on so many metal single-crystal surfaces, though t to arise from C-H ... M interactions which may provide an important dehydrogenation channel. In order to determine the nature of the C-H . .. M interaction, adsorption on oxygen-modified Ni{111} surfaces was a lso investigated. The effect of the coadsorbed oxygen is strongly cove rage dependent; adsorption of cyclohexane on the Ni{111}-(2 x 2-)-O su rface results in further downshifting of the softened nu(CH) stretchin g vibration, signalling the importance of charge transfer from the fil led CH sigma orbital to the metal in weakening the C-H bond. Adsorptio n on the Ni{111}-(root 3 x root 3)R30 degrees-O surface leads to total suppression of any C-H ... M interaction, attributed to steric blocki ng of bare metal sites by the adsorbed adatoms.